J. Am. Chem. SOC. 1992, 114, 1318-1329
1318
Asymmetric Polymerization of Triphenylmethyl Methacrylate Leading to a One-Handed Helical Polymer: Mechanism of Polymerization Tamaki Nakano,+ Yoshio Okamoto,*>+and Koichi Hatadat Contribution from the Department of Applied Chemistry, Faculty of Engineering, Nagoya University, Chikusa- ku, Nagoya 464-01, Japan, and Department of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan. Received March 1, 1991
Abstract: Asymmetric oligomerization of triphenylmethyl methacrylate (TrMA) was carried out with complexes of 9fluorenyllithium and chiral ligands in toluene at -78 "C, and the oligomers obtained were converted into methyl esters. The resulting oligo(methy1methacry1ate)s were first fractionated by gel permeation chromatographyin terms of degree of polymerization and further separated into diastereomers and optical isomers by high-performance liquid chromatography. The distribution of oligomers and the ratio of isomers in each oligomer gave important information on the mechanism of the asymmetric (helix sense selective) polymerization of TrMA. The reactivity of each oligomer anion depended greatly on its degree of polymerization and stereostructure. The oligomer anions whose asymmetric centers have R configuration in the system with the complex of 9-fluorenyllithium and (-)-sparteine and those of S configuration in the systems with (+)-(2S,3S)-2,3-dimethoxy-1,4bis(dimethy1amino)butane and ( + ) - ( S ) -1-(2-pyrrolidinyImethyl)pyrrolidineas chiral ligands predominantly propagated to a one-handed helical polymer. A stable helix starts at degree of polymerization 9 in the former system and at degree of polymerization 7 in the latter two systems. One helix turn seems to consist of three or four monomeric units. The main chain of the resulting polymer in the former system possessed RRR--- absolute configuration and that in the latter systems SSS---, though both polymers are considered to be of the same helicity, P or M . These results indicate that the helicity of the polymer is not governed by the configuration of the main chain but by the chirality of the ligands.
Introduction The helix is one fundamental structure for macromolecules. Many stereoregular macromolecules including naturally occurring and synthetic ones are known to take helical conformation in the solid state.' A polymer with right- or left-handed helical conformation can be optically active without any chiral component because it is chiral. However, most isotactic vinyl polymers such as polystyrene and polypropylene without chiral pendant groups cannot be optically active in solution because the dynamics of polymer chains is extremely fast at room temperature in solution and therefore the polymers cannot maintain a helical conformation.2 However, there exists the possibility of obtaining optically active polymers if the polymer backbone is very rigid or sterical repulsion of side groups is large enough to maintain a stable conformation. These possibilities have been realized in a few synthetic polymers: polyis~cyanides,~ polyis~cyanates,~ poly~ h l o r a l and , ~ poly(triarylmethy1 methacrylate)s6 The first example of this kind of optically active polymer is poly(tert-butyl isocyanide). This was confirmed by chromatographic optical resolution of the polymer synthesized with an achiral initiator system,3a,band recently direct asymmetric synthesis with optically active Ni(I1) complexes was reali~ed.~" The presence of the bulky tert-butyl group appears necessary to maintain the helical conformation. Optically active polyisocyanate can be obtained by anionic copolymerization of achiral isocyanates with a small amount of an optically active i ~ o c y a n a t e .Its ~ optical activity is much greater than the activity of the chiral isocyanate. Onehanded helical structure is induced by incorporation of a small amount of the chiral isocyanate. Although polychloral prepared by an enantiomerically pure initiator is considered to possess one-handed helical conformation, the very large optical activity of the polymer has been confirmed only in film because the polymer is in~oluble.~",~ Several optically active poly(triarylmethy1 methacry1ate)s have been directly synthesized by asymmetric (helix sense selective) anionic polymerization.6 The helices of polychloral and poly(triarylmethy1 methacry1ate)s are considered to be maintained by the steric repulsion between the bulky side groups. Triphenylmethyl methacrylate (TrMA) is the first example of a vinyl monomer which directly affords an optically active, highly isotactic polymer by polymerization with chiral initiators.6a- The 'Nagoya University. 'Osaka University.
optical activity of the polymer arises mainly from a stable onehanded helical conformation because poly(methy1 methacrylate) derived from the poly(triphenylmethy1 methacrylate) (poly(TrMA)) shows very low optical activity. Optically active poly(TrMA) shows high chiral recognition ability as a chiral stationary phase for optical resolution by high-performance liquid chromatography (HPLC), and many racemic compounds have been resolved on the phase.' Therefore, clarification of the (1) (a) Tadokoro, H. Structure of Crystalline Polymers; Wiley: New York, 1979. (b) Vollmert, B. In Polymer Chemistry; Springer-Verlag: New York, 1973; pp 593-597. (2) (a) Frisch, H. L.; Schuerch, C.; Szwarc, M. J . Polym. Sci. 1953, 1 1 , 559. (b) Pino, P. Adu. Polym. Sci. 1965, 4, 393. (3) (a) Nolte, R. J. M.; van Beijnen, A. J. M.; Drenth, W. J . A m . Chem. SOC.1974, 96, 5932. (b) Drenth, W.; Nolte, R. J. M. Acc. Chem. Res. 1979, 12, 30. (c) van Beijnen, A. J. M.; Nolte, R. J. M.; Naaktgeboren, A. J.; Zwikker, J. K.; Drenth, W. Macromolecules 1983, 16, 1679. (d) Green, M. M.; Gross, R. A.; Schilling, F. C.; Zero, K.; Crosby, F., Ill. Macromolecules 1988, 21, 1839. (e) Kamer, P. C.; Nolte, R. J. M.; Drenth, W. J . Am. Chem. SOC.1988, 110, 6818. (4) (a) Green, M. M.; Andreola, C.; Munoz, B.; Reidy, M. P.; Zero, K. J . A m . Chem. SOC.1988, 110, 4063. (b) Green, M. M.; Reidy, M. P.; Johnson, R. J.; Darling, G.; O'Leary, D. A,; Willson, G. J . Am. Chem. SOC. 1989, 1 1 1 , 6452. (5) (a) Corley, L. S.; Vogl, 0. Polym. Bull. 1980, 3, 111. (b) Vogl, 0.; Corley, L. S.; William, J. H.; Jaycox, J. D.; Zhang, J. Makromol. Chem. Suppl. 1985, 13, 1. (c) Zhang, J.; Jaycox, G. D.; Vogl, 0. Polym. J . 1987, 19, 603. (d) Ute, K.; Nishimura, T.; Hatada, K.; Xi, F.; Vass, F.; Vogl, 0. Makromol. Chem. 1990, 191, 557. (6) (a) Okamoto, Y.; Suzuki, K.; Ohta, K.; Hatada, K.; Yuki, H. J . A m . Chem. SOC.1979, 101, 4796. (b) Okamoto, Y.; Suzuki, K.; Yuki, H. J . Polym. Sci. Polym. Chem. Ed. 1981, 18, 3043. (c) Okamoto, Y.; Shohi, H.; Yuki, H. J . Polym. Sci. Polym. Lett. Ed. 1983, 21, 601. (d) Okamoto, Y.; Mohri, H.; Hatada, K. Chem. Lett. 1988, 1879. (e) Okamoto, Y.; Yashima, E.; Hatada, K. J . Polym. Sci. Polym. Lett. Ed. 1987, 25, 297. (f) Okamoto, Y.; Nakano, T.; Asakura, T.; Mohri, H.; Hatada, K. J . Polym. Chem. Part A : Polym. Chem. Ed. 1991, 29, 287. (g) It was reported that oligo- and polymethacrylates such as poly(MMA) and poly(benzy1 methacrylate) maintain their helical conformation in solution: Cram, D. J.; Sogah, D. Y. J . A m . Chem. Soc. 1985, 107, 8301. However, we believe that such ester groups are not bulky enough to maintain their helical conformation: Okamoto, Y.; Nakano, T.; Hatada, K. Polym. J . 1989, 21, 199. (7) (a) Yuki, H.; Okamoto, Y.; Okamoto, I. J . Am. Chem. SOC.1980, 102, 6356. ( b ) Okamoto, Y.; Honda, S.; Okamoto, I.; Yuki, H.; Murata, S.; Noyori, R.; Takaya, H. J . Am. Chem. SOC.1981, 103, 6971. (c) Okamoto, Y.; Okamoto, I.; Yuki, H. Chem. Lett. 1981, 853. (d) Okamoto, Y.; Yashima, E.; Ishikura, M.; Hatada, K. Bull. Chem. SOC.Jpn. 1988, 61, 255. (e) Okamoto, Y.; Yashima, E.; Hatada, K.; Mislow, K. J . Org. Chem. 1984, 49, 557. (f) Chance, J. M.; Geiger, J. H.; Okamoto, Y.; Aburatani, R.; Mislow, K. J . Am. Chem. SOC.1990, 112, 3540. (g) Okamoto, Y.; Hatada, K. J . Liq. Chromatogr. 1986, 9 , 369.
0002-7863/92/ 15 14-13 18$03.00/0 0 1992 American Chemical Society
Polymerization of Triphenylmethyl Methacrylate
J . Am. Chem. SOC.,Vol. 114, No. 4, 1992 1319
detailed mechanism of this unique asymmetric polymerization is an attractive and challenging problem. Previously, we reported the preliminary results of asymmetric oligomerization of TrMA with a complex of 9-fluorenyllithium (FlLi) and (-)-sparteine (Sp) as an initiator.6 TrMA gives an optically active polymer with several chiral initiators such as the complexes of S p and butyllithium (n-BuLi),6a,bS p and F1Li,8 Sp and (1,l-diphenylhexy1)lithium (DPHLi),8s9aand (+)-(2S,3S)- or (-)-(2R,3R)-2,3-dimethoxy- 1,4-bis(dimethylamino)butane (DDB) and (N,N'-di-
H PMP
oligo(MMA)
(n=2-8)
MMA dimer having a fluorenyl group at the a end derived from (2R,4R)-2,4-dimethylglutaric acid.
Experimental Section poly(TrMA)
phenylethy1enediamine)monolithium amide (DPEDA-Li).6c However, in most cases, the products of the polymerization were a mixture of a polymer (80-90 wt 7' %) of high optical activity and an oligomer (10-20 wt %) of low optical activity. The oligomer of low optical activity was considered to be produced from the species with lower activity than the species for the one-handed helical polymer; that is, the oligomer anions of certain specific stereostructure would propagate to the polymer, and the others would remain as oligomers until completion of the polymerization.6b Therefore, the composition of stereoisomers in the oligomer anions should change in the process of polymerization. Wulff and co-workers also reported similar oligomerizations of TrMA with S p D P H L i 9 " and Sp(diphenylmethy1)lithium (DPMLi) comp l e ~ e s .They ~ ~ separated the oligomers in terms of degree of polymerization (DP) and analyzed the oligomers (DP = 1-4) by 'H and I3C N M R spectroscopies as a mixture of diastereomers. In the present paper, we report the detailed results of the asymmetric oligomerization of TrMA and the complete separation and assignment of resulting oligomers. With the obtained results, the mechanism of the asymmetric polymerization of TrMA is discussed in detail. Oligomerization of TrMA was carried out by the complexes of FlLi with three chiral ligands in toluene at -78 "C at the several [TrMA]/[Li] ratios and terminated by protonation with C H 3 0 H to give the oligomers having a fluorenyl group at the initiation end (a end) and a hydrogen at the termination end (w end).1° The chiral ligands employed were Sp, DDB, and (+)-(S)-1-(2-pyrr0lidinylmethyl)pyrrolidine~~(PMP). The resulting oligomers were converted into their methyl esters (oligo(MMA)) and fractionated by gel permeation chromatography (GPC) in terms of DP. Each oligomer was further separated into diastereomers and optical isomers by HPLC using columns packed with silica gel and polysaccharide derivative coated silica gel, respectively. The assignments of diastereomers were accomplished by 'H N M R analyses. The absolute configurations of oligomers were determined on the basis of an optically active (8) Okamoto, Y . ;Yashima, E.; Nakano, T.; Hatada, K. Chem. Lett. 1987, 759. (9) (a) Wulff, G.; Sczepan, R.; Steigel, A. Tetrahedron Lett. 1986, 27, 1991. (b) Wulff, G.; Vogt, B.; Petzoldt, J. ACS Polym. Mar. Sa'. Eng. 1988, 58, 859. (10) According to the IUPAC structure-based nomenclature for polymers, the polymer obtained in the present study is named a-hydro-w-9-fluorenyl~ polymethacrylate in which the process of the formation of polymer chain is irrespective. However, in the present paper, the CY and w ends are designated as prefixes of the beginning (the side of fluorenyl group) and the terminal (the side of methine hydrogen originated from termination reagents) of the chain, respectively, on the basis of the formation process of the polymer.
Materials. Toluene was purified in the usual manner, mixed with a small amount of n-BuLi, and distilled under high vacuum just before use. Tetrahydrofuran (THF) was refluxed over CaH2 and distilled over LiAlH,. n-BuLi was synthesized from butyl chloride (BuCI) and Li powder in heptane under argon atmosphere" and was used as a 0.756 M solution for preparation of an initiator solution. Fluorene (Nacalai Tesque) was first recrystallized from ethanol and then from hexane; mp 104.5-105.0 OC. Chiral ligands, Sp (Sigma), (+)-DDB (Aldrich), and P M P (Aldrich), were dried over CaH2 and distilled under reduced pressure. TrMA was synthesized from methacrylic acid and triphenylmethyl chloride in the presence of triethylamineI2 and was first recrystallized from diethyl ether and then from hexane; mp 101.9-102.9 OC (lit.13 mp 102-103 "C). Oligomerization and Polymerization Procedure. FlLi was prepared by adding 1 equiv of n-BuLi to a solution of fluorene in toluene at room temperature. This was mixed with 1.2 equiv of a chiral ligand. The mixture was left for 10 min at room temperature for the formation of a complex. The oligomerization was carried out in a dry glass ampule under a dry nitrogen atmosphere. TrMA (1.0 g, 3.05 mmol) was placed in a glass ampule, which was then evacuated on a vacuum line and flushed with dry nitrogen. After this procedure was repeated three times, a three-way stopcock was attached to the ampule and toluene or T H F (20 mL) was added with a hypodermic syringe to dissolve TrMA. Then, the monomer solution was cooled to -78 OC, and a prescribed amount of an initiator solution was added to the monomer solution with a syringe. The reaction was terminated by the addition of a small amount of C H 3 0 H . After termination, the solvent was evaporated and a part of resulting oligomer was solvolyzed by refluxing in C H 3 0 H containing a small amount of hydrochloric acid. The resulting oligo(methacry1ic acid) was suspended in benzene and methylated by CH2N2in ether solution to give oligo(MMA).', Polymerization was carried out in the same way as the oligomerization described above. The polymerization was also done in a quartz optical cell to monitor the optical activity of the reaction system.6c In the case of polymerization, the products were poured into a large amount of C H 3 0 H after termination and collected by centrifugation as quickly as possible. The polymer was dried under high vacuum at 60 OC for 3 h. The polymer was once dissolved in T H F or in a mixture of T H F and CH2Br2and poured into a large amount of a mixture of benzene and hexane ( l / l , v/v). The insoluble part was collected by centrifugation and the soluble part by evaporation under high vacuum. Conversion of the polymer into the methyl ester (poly(MMA)) was done in the same way as applied for the oligomers. Preparation of MMA Dimer Having a 9-Fluorenyl Group at the a End from 2&Dimethylglutaric Acid. 9-(Iodomethyl)fluorene was synthesized from 9-fluorenylmethanol. 9-Fluorenylmethanol (Aldrich; 4.98 g, 25.4 mmol) and aqueous H I (Nacalai Tesque; 55% v/v, 16.0 mL) were mixed in a round-bottomed flask, and the resultant mixture was heated to 90 O C with vigorous stirring for 26 h. A deep red mixture was extracted with diethyl ether. The ethereal layer was washed with water, dried over magnesium sulfate, and evaporated under high vacuum. The crude product was a mixture of the iodide and the unreacted alcohol at a molar ~~
(11) Ziegler, K.; Gellert, H. G. Ann. 1950, 567, 179. (12) Yuki, Y. Japan Kokai 560-8117, 1981. (13) (a) Adrova, N. A,; Prokhorova, L. K. Vysokomol. Soedin. 1961, 3, 1509. (b) Okamoto, Y.; Yuki, H. In Macromolecular Syntheses; Stille, J. K., Ed.; Robert E. Kreiger Publishing: FL, 1990, Vol. 10, pp 41-44. (14) Katchalski, A.; Eisenberg, H. J . Polym. Sci. 1951, 6, 145.
Nakano et al.
1320 J . Am. Chem. SOC.,Vol. 114, No. 4, 1992 Table I . Asvmmetric Polvmerization of TrMA with SD-. DDB-. and PMP-F1Li ComDlexes in Toluene at -78 B/Hb-insoluble part [e]' x 10-4 [e]. x 10-5 run initiator time (h) yield' (%) yield (%) [ a ] 2 5 D(deg) d (235 nm) (210 nm) +2.32 82 +383 +9.42 1 SpFlLi 24 99 +1.86 93 +344 +8.45 24 100 2 DDB-F1Li +7.78 1.76 3 100 94 +334 3 PMP-FILi
OC"
~~
M,/M,/ 1.31 1.10 1.12
DPI 60 47 39
+
tacticityg (%) m m >99h >99h >99h
"Conditions: TrMA, 1.0 g (3.05 mmol); toluene, 20 mL; [TrMA]/[Li] = 20. * A mixture of benzene and hexane ( l / l , v/v). 'CH30H-insoluble part. d~ 0.5 (THF). 'CD spectrum was measured in T H F at ca. 25 "C. Units: cm2 dmol-I. (Determined by GPC of poly(MMA) derived from poly(TrMA). gDetermined by ' H N M R of poly(MMA) derived from poly(TrMA). *The signals due to the racemo sequence were found only in the w end.
Chart I. Structure and Numbering System of Monomeric Units of Oligo- and Poly(MMA) j a, j a2 j a3 j j " 3 j
@'C H 2 - CF- CHHs 2i - C - C H 2F- CH 3 c.0
c=o
7H3
y
j
7H3 7H3 CH2-C-CH2-C-CHp-C-H
"
j
1
7th 0
c-0
ratio of 52/48 by 'H N M R analysis. The alcohol was removed by silica gel column chromatography with a mixture of diethyl ether and hexane ( l / l , v/v) as eluent to give red crystals. The crystal obtained was dissolved in diethyl ether. The solution was washed with a saturated water solution of Na2S0, to remove iodine and dried over MgS04 to give 2.48 g (31.9%) of the iodide free from alcohol and iodine. Further purification was done by recrystallization twice from diethyl ether and once from hexane to give 0.61 g (7.9%) of white crystals: mp 89.7-90.7 OC; ' H N M R (270 MHz, CDCI,, Me4Si) 6 7.30-7.75 (m, 8 H , aromatic), 4.17 (t, 1 H, CH), 3.71 (d, 2 H , CHI). Anal. Calcd for C14HilI: C, 54.93; H , 3.62; I, 41.45. Found: C, 54.97; H, 3.67; I, 41.26. 2,4-Dimethylglutaric acid (Aldrich; a mixture of (i) and meso isomers) was methylated with CH2N2. The resulting dimethyl 2,4-dimethylglutarate was reacted with 1 equiv of lithium diisopropylamide (LDA) in dry T H F at 0 OC for 0.5 h under a dry nitrogen atmosphere in a round-bottomed flask equipped with a three-way stopcock. Then, 1 equiv of 9-(iodomethyl)fluorene in dry T H F was added with a syringe. The solution was stirred for 3 h at 0 OC. A large excess of hydrochloric acid in CH,OH was then added to the solution. After the solvent was evaporated, the crude product was extracted with diethyl ether. The ethereal layer was washed with water and dried over MgSO,. H P L C separation of the product was done by using an HPLC column (50 X 0.72 (i.d,) cm) packed with silica gel (Nomura Chemicals, Develosil 100-5). A mixture of BuCl and CH,CN (97/3, v/v; flow rate 2.4 mL/min) was used as an eluent.8.1s Preparation of Optically Active MMA Dimer Having a 9-Fluorenyl Group at the a End from (-)-(2R,4R)-2,4-Dimethylglutaric Acid. 2,4Dimethylglutaric acid (a mixture of (i) and meso isomers) was converted into its acid chloride with 1.2 equiv of S0Cl2,and the product was distilled under reduced pressure to give 2,4-dimethylglutaroyI dichloride, bp 77-85 OC (3 mmHg). The acid chloride was reacted with benzyl alcohol in the presence of excess triethylamine to give the dibenzyl ester, which was separated into optical isomers by HPLC. Optical resolution was done with a chiral H P L C column (50 X 2.0 (i.d.) cm) packed with amylose-tris( 3,5-dimethylphenylcarbamate)-coatedmacroporous silica by using a mixture of hexane and 2-propanol (98/2, v/v; flow rate 9.9 mL/min) as eluent. The (-) isomer eluted at 27.2 min, the meso isomer at 29.0 min, and the (+) isomer at 3C.4 min. The (-) isomer ([(Y]"D -31O (c 2.54, hexane)) was hydrolyzed by hydrochloric acid to give an optically active acid: [(u]z5D-21' (c 0.88, CHIOH) (lit." [(YIz5D -40' (water)). Then, the optically active acid was methylated with CH,N2, giving the dimethyl ester, [.Iz5D -31' (c 0.93, CH,OH). This optically active (R,R)-dimethyl ester was used for preparation of the M M A dimer in the same way as described above for the synthesis of the racemic M M A dimer. Measurements. ' H N M R spectra were measured on JEOL GX-500 (~OO-MHZ),JEOL GSX-270 (270-MHz), and Varian VXR-500 (500MHz) spectrometers. Measurements were done in CDCl3 at 35 OC or in nitrobenzene-d5at 110 OC. Two-dimensional spectra were taken under the same spectral conditions as described before.18 Optical rotation was measured with a Jasco DIP-181 polarimeter. Circular dichroism (CD) (15) Andrews, G. D.; Vatvars, A. Macromolecules 1981, 14, 1603. (16) Okamoto, Y.; Aburatani, R.; Fukumoto, T.; Hatada, K. Chem. Lett. 1987, 1857. (17) Fredga, A. Arkio Kemi 1947, 24A, No. 32.
spectra were obtained with a Jasco 5-720 spectrometer. Field desorption (FD) mass spectra were measured on a JEOL DX-HF303 spectrometer. The molecular weights of the polymers were determined by GPC measurement of poly(MMA) derived from the original polymer on a Jasco Trirotar IIP chromatograph equipped with a Jasco RI-SE64 (reflective index) detector. Two commercial columns (Shodex KF-802.5, 30 X 0.72 (i.d.) cm; Shodex AC-IOM, 50 X 0.72 (i.d.) cm) were connected in series, and CHCI, was used as the eluent. A calibration curve was obtained with standard polystyrene. Separation by H P L C was done by using Jasco Trirotar I1 and BIP-I chromatographs equipped with one or two of the following: Jasco UVIDEC-100-111 (UV), UVIDEC-100-V (UV), MULTI-320 (UV), and DIP-I 8 1C (polarimetry) detectors. An automatic eluent mixer, Jasco GR-ASO, was employed for the diastereomeric separation of oligomers. Columns used and chromatographic conditions were as follows. For the GPC analysis of the oligomers, a column packed with poly(styrene-cop-divinylbenzene) gel (50 X 2.2 (i.d.) cm, maximum porosity 3000) or two commercial columns connected in series (Shodex Gel-101, 50 X 0.72 (i.d.) cm) were used with CHCI, as an eluent (flow rate 3.0 mL/min for the former column and 0.5 mL/min for the latter ones). For the separation of diastereomers, columns (25 X 0.46 (i.d.) cm, 50 X 0.72 (id.) cm) packed with silica gel (Nomura Chemicals, Develosil 100-5) were used with a mixture of BuCl and CH3CN as the e l ~ e n t *in~ programmed '~ ratios: from 95% BuCl to 60% BuCl during a 60-min period (flow rate 0.5 mL/min for the former column and 2.4 mL/min for the latter column). Optical resolution of oligo(MMA)s was done on chiral columns (25 X 0.46 (i.d.) cm) packed with cellulose derivatives using a hexanealcohol eluting system (flow rate 0.5 mL/min)." For optical resolution of the M M A dimer prepared from 2,4-dimethylglutaric acid, a column packed with cellulose-tris(3,5-difluorophenylcarbamate)-coatedmacroporous silica was used with a mixture of hexane and 2-propanol (95/5, v/v) as the eluent. For optical resolution of the M M A dimer obtained from oligomerization systems, a column packed with cellulose-tris(3,5-dichlorophenylcarbamate)-coated macroporous silica geli9b was used with a mixture of hexane and 2-propanol (95/5, v/v) as the eluent. For optical resolution of the mixture of M M A trimers mm and rm, a column packed with cellulose-tris(3,4-dichlorophenylcarbamate)-coated macroporous silica gel'9bwas used with a mixture of hexane and 2-propanol (90/10, v/v) as the eluent. For optical resolution of M M A trimers rr and mr, a column packed with cellulose-tris(3,5dichlorophenyl/dimethylphenylcarbamate)-coatedmacroporous silica gel'9bwas used with a mixture of hexane and 2-propanol (90/10, v/v) as the eluent. For optical resolution of the M M A pentamer, hexamer, heptamer, or octamer, a column packed with cellulose-tris(3,5-dimethylpheny1carbamate)-coatedmacroporous silica geligbwas used with a mixture of hexane and ethanol (80/20, V/V)as the eluent. (18) (a) Ute, K.; Nishimura, T.; Matsuura, Y.; Hatada, K. Polym. J . 1989, 21, 231. (b) Ute, K.; Nishimura, T.; Hatada, K. Polym. J . 1989, 21, 1027. (c) Hatada, K.; Ute, K.; Tanaka, K.; Kitayama, T. Polym. J . 1987, 19, 1325. (d) Hatada, K.; Ute, K.; Tanaka, K.; Imanari, M.; Fujii, N . Polm. J . 1987, 19, 425. (19) (a) Okamoto, Y.; Kawashima, M.; Hatada, K. J . Am. Chem. SOC. 1984, 206, 5357. (b) Okamoto, Y.; Kawashima, M.; Hatada, K. J . Chromatogr. 1986, 363, 173. (20) Okamoto, Y.; Aburatani, R.; Hatada, K. Bull. Chem. SOC.Jpn. 1990, 63, 955.
Polymerization of Triphenylmethyl Methacrylate
J . Am. Chem. Soc., Vol. 114, No. 4, 1992 1321
m a i n chain-CH3
/
I
2.5
,t
I
2.0
1.5
6
t
(")
I
1.0
(ppm)
Figure 1. 500-MHz 'HN M R spectrum of poly(MMA) derived from benzene-hexane-insoluble poly(TrMA) of DP = 60 (1 in Table I) (nitrobenzene-d,, 110 "C). c and X denote I3C satellite bands of the main-chain CH, signal and impurity, respectively.
Results and Discussion Asymmetric Polymerization of TrMA with Three Initiator Systems. The results of asymmetric polymerization of TrMA with Sp-FlLi, DDB-FlLi, and PMP-FlLi are shown in Table I. The three initiators gave highly isotactic, optically active polymers which showed almost the same positive rotation, indicating that the polymers possess a one-handed helical conformation of the same screw sense. The polymers obtained with the three initiators showed large positive CD absorption bands with identical spectral patterns, which were similar to that of the poly(TrMA) prepared with the Sp-n-BuLi complex.6b This also indicates that the three polymers possess the same helicity. The CD spectra showed two peaks at 235 and 210 nm, which may be ascribed to the absorption based on carbonyl and phenyl groups, respectively. The molecular ellipticity ( [ e ] ) values are shown in Table I. The ellipticity values are approximately proportional to the specific rotation of the polymers. Triad tacticity of the polymer was determined by 'H N M R of poly(MMA) derived from the original polymer. As an example, the spectrum of the poly(MMA) of DP 60 derived from the poly(TrMA) obtained with S p F l L i (run 1 in Table I) is shown in Figure 1. In the spectrum, most peaks could be assigned to those of isotactic sequence including the methyl groups in the vicinity of the a and w ends. Racemo sequence was obviously found only for the w end. Assignments of the small peaks in the a-methyl region were done on the basis of detailed studies on the 'H NMR assignments of isotactic oligo- and poly(MMA) having a tert-butyl group at the a end'* and the assignments of isotactic oligo(MMA)s having a 9-fluorenyl group at the a end which will be described later. The structure and the numbering system of the monomeric units of the oligo- and poly(MMA) are illustrated in Chart I. Slightly lower isotacticity of poly(TrMA) in the previous repodaSbmay be due to the fact that no correction was made for the end groups of the polymer chain. GPC curves of poly(TrMA)s obtained with S p and DDB complexes showed two peaks.6b However, poly(MMA) derived from these polymers showed only one GPC peak with a narrow distribution. Part of poly(TrMA) may exist in association form as in the case of poly(dipheny1-2-pyridylmethyl methacrylate) .6d
20
25
30
35
e l u t i o n t i m e (min) Figure 2. G P C curves of oligo(MMA)s derived from oligo(TrMA)s prepared with Sp-F1Li at [TrMA]/[Li] = 2 (A), 3 (B), 5 (C), 10 (D), and 20 (E).
In all the polymerizations shown in Table I, the products could be separated into two fractions: a polymer of high optical rotation which was insoluble in a mixture of benzene and hexane ( l / l , v/v) and an oligomer of low optical activity which was soluble in the solvent as mentioned in the previous paper.6b The amount of oligomer was larger and DP of the polymer was higher in the system with Sp-F1Li than in the systems with DDB-F1Li and PMP-F1Li at the same [TrMA]/[Li] ratio. In the system with S p F l L i , the relative amounts of less active oligomer anions may be larger and those of active oligomer anions which can propagate to the polymer may be smaller. This should result in the formation of a polymer of higher DP. It has been reported that the polymerization of TrMA with DDB-n-BuLi is much faster than that with Sp-n-BuLi.6C This was based on measurement of the change of optical activity of the polymerization system during reaction. Optical activity of the systems increased with polymerization time and reached a final constant value within 16 h after initiation in the system with Sp-n-BuLi and within 2 h in the system with DDB-.n-BuLik The rate of polymerization with PMP-FILi was examined in the present study in the same way as described before.6c The optical rotation reached at a large positive value (a-78D +3.2') within 10 min. The value is comparable to the final values observed in the systems with Spn-BuLi and DDB-n-BuLi.6C The rate of polymerization with PMP-FlLi may be much higher than those with Sp-FlLi and DDB-FlLi. Distribution of Oligomers. In all the oligomerizations, the reaction of TrMA with the initiators seems to proceed almost quantitatively to give oligomers with a 9-fluorenyl group at the a end and a hydrogen at the w end because no clear sign of unreacted monomer and side products which might be produced by the attack of the carbonyl group of TrMA with FlLi was found on the IR and 'H N M R spectra of reaction mixtures. GPC analysis also supported this. Figures 2-4 show the GPC curves of oligo(MMA)s derived from oligo(TrMA)s prepared with Sp-FILi, DDB-FlLi, and PMP-FlLi as initiators, respectively, at [TrMA]/[Li] = 2, 3, 5, 10,and 20. In these GPC curves, the ratio of the peak intensity approximately corresponds to the molar ratio of oligomers because UV detection at 254 nm is mainly due to a fluorenyl group at the CY end of each oligomer. The chromatographic patterns of the original oligo(TrMA)s were similar to those of the corresponding
Nakano et al.
1322 J. Am. Chem. Soc., Vol. 114, No. 4, 1992
Chart 11. Intramolecular Solvation of the w-End Anion
30-20 I
6 5 4
2 L
I
I
25
20
3
30
35
40
elution time (min) Figure 3. G P C curves of oligo(MMA)s derived from oligo(TrMA)s prepared with DDB-FILI at [TrMA]/[Li] = 2 (A), 3 (B), 5 (C), 10 (D), and 20 (E).
30-20 25
30
65 4
35
3
2
40
weights of the oligomers of DP 2-8 with the structure shown in Chart 1. The spectrum also showed another series of smaller peaks of m/z 350,450, 550,650, 750, 850, and 951, though the definite structure of the oligo(MMA)s corresponding to these molecular weights is unknown at the present time. Oligo(MMA)s with a structure other than that shown in Chart I may be formed in the process of conversion of oligo(TrMA) into methyl esters.*’ The intensity ratio of the main peaks was similar to that of the GPC curve. In the GPC curves, the peak corresponding to a unimer (DP 1 ) was not clearly observed except for oligo(MMA) obtained with PMP-F1Li at [TrMA]/[Li] = 2 and, even in that case, it was much smaller than the other peaks. The existence of a unimer was not confirmed even in the equimolar reaction of TrMA and SpFlLi,* indicating that the unimer anion is much more reactive than the initiator complex and dimer anion. The high activity of the unimer anion may be due to the effect of “intramolecular solvation” proposed for anionic polymerization of methacrylates.22 As illustrated in Chart 11, the wend anion of the propagating species is stabilized by complex formation between the Li cation and the carbonyl group of the penultimate monomeric unit. This stabilization is not possible for 2 unimer anion. On the other hand, Wulff and nworkers reported that a significant amount of unimer was found in the system with Sp-DPHLi at [TrMA]/[Li] = 59a and in the systems with Sp-DPMLi at [TrMA]/[Li] = 0.5 and l?b The activity of the unimer anion in their systems may be much lower than that in our systems. The a end group of an oligomer anion may affect its activity in the oligomerization. The oligomer distributions at [TrMA]/[Li] = 5 in all the systems were not simple. Although the oligomers of DP
